BIP 433: Add P2A BIP

[instagibbs]

For interoperability with other schemes, it behooves me to write a BIP for this output type. It's already standard to spend in well over half the network.

h/t roasbeef for bothering me about this repeatedly

[Roasbeef]

This BIP has been assigned number 433!

(can update the readme in the PR to reflect assignment)

[murchandamus]

Please add an entry to the table in the README.md for your BIP, add the number to the preamble, and update the “Created” header to 2025-12-08.

[instagibbs]

Thanks for the review and number!